This invention relates to wire coiling devices, and, more particularly, to a device for longitudinally twisting wire and then forming the twisted wire into coils of precise diameter and pitch.
Coil springs, both of the compression and extension type, have been formed in prior art coiling machines by first wrapping a length of wire around a mandrel. The wire is stretched beyond its elastic limit as it is wrapped around the mandrel, so that the outer edge of the coil is longitudinally stretched in tension and the inner edge of the coil is stressed in compression. Once coiled, the wire must then be compressed in length to form a finished pre-stressed compression or extension spring. In many prior art machines it was necessary to form the wire in coils approximately six inches long to obtain a finished spring three inches long. Compression of the originally wound coils also changes the diameter of the coils in the finished spring, and this had to be taken into account in forming coil springs in accordance with such prior art methods.
As discussed in detail in U.S. Pat. No. 3,541,828 to Norman, a disadvantage of coil springs formed by the above-described technique is that the maximum working stress or stiffness of such springs is limited. The limited working stress results from forming the spring by first winding it into a coil and then compressing the coils into a finished spring. It has been found that a greater circumferential prestress can be applied to wire by first longitudinally twisting the wire beyond its elastic limit and then forming the coils, rather than twisting the wire beyond its elastic limit in the formation of coils. The greater the circumferential pre-stress in the wire the greater the stiffness or working strength. In addition, the longitudinal tension stresses applied to the outer surface of the coils formed with the prior art method can lead to surface fractures of the coil and subsequent breakage of the spring.
In the Norman coiling device, the wire is first twisted longitudinally beyond its elastic limit and then formed in a coil of the desired diameter and length. The coils need not be compressed to form the finished spring and therefore do not change in diameter. Additionally, as discussed above, the working stress or stiffness of a spring formed by the Norman coiling device is greater than a spring of the same diameter formed by the prior art method because a greater circumferential pre-stress can be applied to the wire by twisting it before it is formed into a coil.
The Norman coiling device employs a planetary gearing arrangement to form coils from pretwisted wire. A feed wheel mounted on a rotating platen is movable in a planetary motion with respect to the platen. The feed wheel receives the wire from a twisting device which imparts a longitudinal twist to the wire, and the feed wheel then guides the twisted wire to a stationary stanchion having coil-forming rollers mounted thereto. The twisted wire is held by a belt against the feed wheel as it moves from the twisting device to the stanchion for coiling.
One problem with the Norman coiling device has been the formation of coils of non-uniform diameter and/or pitch. This is attributable to the manner in which the twisted wire is held or secured to the feed wheel in moving from the twisting device to the coil-forming rollers on the stanchion. It has been found that the wire slips between the belt and the feed wheel so that the length of wire supplied to the coil-forming rollers can vary. This results in the formation of coils having different diameters. The inability of the belt to secure or clamp the wire against the feed wheel is a particular problem with heavy gauge wire where a substantial force is required to both twist the wire longitudinally and form it in a coil on the stanchion.
One solution to the problem of wire slippage at the point where it is formed into coils is found in German Pat. No. 2,700,924. In this coiling device, three drive rollers each having wire-engaging grooves are spaced approximately 120.degree. apart about the periphery of a mandrel. Wire fed from a reel is clamped between the grooves in the drive rollers and the mandrel to positively grip the wire and avoid slippage. The drive wheels are rotated with respect to the fixed mandrel to bend the wire against the mandrel to form a coil, and then to advance the coils along the mandrel at a controlled rate.
Although the German coiling device solves the wire gripping problem of the Norman apparatus, it has several limitations. The German device produces coils in the same manner as the prior art described above wherein the wire is not twisted prior to the formation of coils. This limits the stiffness or working stresses in the springs produced by the German device, and creates problem of surface fractures in the coils, as discussed above. Additionally, the pitch of the coils is formed in the German device by a machined groove in one of the drive rollers. The pitch cannot be adjusted without removing the drive roller and replacing it with another drive roller machined with a different pitch-forming groove.